TCP/IP model: Difference between revisions

Revision as of 07:27, 27 March 2012

Revision as of 07:31, 27 March 2012

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==OSI and TCP/IP layering differences==

The three top layers in the OSI model—the [[application layer]], the [[presentation layer]] and the [[session layer]]—are not distinguished separately in the TCP/IP model where it is just the Applicationapplication layer. While some pure OSI protocol applications, such as [[X.400]], also combined them, there is no requirement that a TCP/IP protocol stack must impose monolithic architecture above the transport layer. For example, the [[Network File System]] (NFS) application protocol runs over the [[External Data Representation|eXternal Data Representation]] (XDR) presentation protocol, which, in turn, runs over a protocol called [[Remote Procedure Call]] (RPC). RPC provides reliable record transmission, so it can run safely over the best-effort [[User Datagram Protocol]] (UDP) transport.

The session layer roughly corresponds to the Telnet [[virtual terminal]] functionality{{Citation needed|date=June 2009}}, which is part of text based protocols such as the [[HTTP]] and [[SMTP]] TCP/IP model application layer protocols. It also corresponds to TCP and UDP port numbering, which is considered as part of the transport layer in the TCP/IP model. Some functions that would have been performed by an OSI presentation layer are realized at the Internet application layer using the [[MIME]] standard, which is used in application layer protocols such as [[HTTP]] and [[SMTP]].

The IETF protocol development effort is not concerned with strict layering, and some of its protocols may not fit cleanly into the OSI model. These conflicts, however, are apparent also in the original OSI model, ISO 7498, when not considering the annexes to this model (e.g., ISO 7498/4 Management Framework), or the ISO 8648 Internal Organization of the Network layer (IONL). When the IONL and Management Framework documents are considered, the [[Internet Control Message Protocol|ICMP]] and [[Internet Group Management Protocol|IGMP]] are neatly defined as layer management protocols for the network layer. In like manner, the IONL provides a structure for "subnetwork dependent convergence facilities" such as [[Address Resolution Protocol|ARP]] and [[Reverse Address Resolution Protocol|RARP]].

IETF protocols can be encapsulated recursively, as demonstrated by tunneling protocols such as [[Generic Routing Encapsulation]] (GRE). While basic OSI documents do not consider tunneling, there is some concept of tunneling in yet another extension to the OSI architecture, specifically the transport layer gateways within the International Standardized Profile framework.<ref>[http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=30726 Framework and taxonomy of International Standardized Profiles], ISO 10000, October 1998</ref> The associated OSI development effort, however, has been abandoned given the overwhelming adoption of TCP/IP protocols.